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Vintage Diesels, Part 2

Tests and inspections for an old oil burner

Part one of these articles about evaluating diesel engines ended by discussing the principles and the importance of performing an informal compression test. Let's now move on to how that's actually done, and then let's look at some additional tests and inspections which can show a complete picture of the state of the engine in the vehicle you are considering for purchase (or perhaps currently own).

Compression Test

The engine will need to spin for at least three to five seconds in order to properly execute this test. Chances are, unless something is done to avert it, the engine will actually start during this time. To prevent this from happening, you could disconnect the fuel supply to the injector pump, but it could get messy if you're not careful, plus it'll take a while to purge the air that the pump will ingest before the engine can be restarted.

A much simpler way to do this (here's where the cold engine helps) is to crank the engine before the glow plugs heat up. You'll get your three to five seconds, and won't have a mess all over the driveway. Although I wouldn't recommend this method frequently of starting an older diesel engine, a couple of times won't hurt. All you need to do is turn the ignition key immediately to the start position (as in, not waiting for the glow plug pilot lamp to cycle on and off like you would normally do when starting the engine) and observe the sound it makes while cranking.

Now, the sound that should be heard on this particular engine is sort of like a ruhhh-ruhhh-ruhhh-ruhhh-ruhhh, indicating even compression. Unfortunately, the Mercedes-Benz we checked exhibited more of a ruhhh-ruhhh-ruhhh-ruhhh-reehhw, indicating low compression in one of the cylinders (and reduced cranking resistance, which altered the pitch of the starter as it was spinning the engine).

Warm-Up Lap

In order to get the complete picture of the engine's condition, you'll need to go ahead and start the engine and take the car for a drive to warm it up. This gives you an opportunity to notice any noises or other drivability problems relating to the engine as well as the rest of the drivetrain and chassis.

When you return, let the engine idle for a minute or two while checking for excessive smoke from the tailpipe, fluid leaks, and any odd noises from bad injectors or other mechanical concerns. Our test engine was okay in the noise department, but exhibited a rather large fluid leak under the engine. It turned out to be a leaking fuel-injection primer pump—not a big deal.

The engine also rocked from side to side considerably while idling, although not actually misfiring. A bad fuel injector can cause this, but here we had to blame the low compression on the one cylinder. Now, THAT'S a big deal.

Torque Converter Test

You'll also want to check the operation of the automatic transmission's torque converter after performing these inspections. Find a flat stretch of clean asphalt with some room to move the vehicle fore and aft. To perform this test, first verify that the parking brake is working by applying it, then attempt to move the vehicle with the transmission in "D." Use relatively light throttle input at first, gradually increasing it until the vehicle moves, or you reach about half-throttle.

If it holds reasonably well (the vehicle does not move forward until you reach about half-throttle), and no abnormal conditions are apparent (like broken engine or transmission mounts—identified by exaggerated engine movement during loading), then shift the transmission to "P" and put a block in front of one of the front tires. Now proceed to the next step.

With the parking brake still engaged, put your foot securely on the service brake, and select the "D" transmission position again. Now you will need to push the accelerator pedal to the floor (full throttle) and observe the engine RPM. It's almost essential that the vehicle have a tachometer, although a good ear will suffice. If you have access to a portable diesel tachometer, using it will work better than your ear.

Observe the tachometer reading, while listening to the engine and transmission. If the RPM hangs around 1,500 or so, the rear tires appear to be trying to spin—even against your braking efforts, and there are no abnormal noises coming from the engine or transmission, then you are probably okay.

If the engine races above 2,000 RPM, and you notice a pronounced shudder from the drivetrain (not wheelspin), or if there are any other obvious whining, squealing, grating or knocking noises coming from the transmission, then there is probably a problem with the torque converter at the very least. Checking the color and condition of the fluid on the dipstick can help support your findings. If it is extremely dark and smells of heat damage, then the transmission and/or the torque converter are not sound. After having checked the engine and transmission while idling, shut the engine off. If it is reluctant to do so, there is probably a problem in the circuit that controls this function (Mercedes-Benz uses a vacuum control for this).

Blow-By Check

If it shuts off with no problem, remove the oil filler cap and observe the intensity of the blow-by fumes. Blow-by is combustion (ignited) fuel that has "blown-by" the piston rings and into the crankcase. There will always be some present, in the form of a white vapor. Prodigious amounts, however, indicate a mechanical problem such as excessive wear to pistons, rings, or cylinders—or it could be a sign from overheating or debris ingestion.

A way to further verify whether the blow-by being generated is beyond the norm is to remove the air cleaner cover and observe engine oil deposits within the housing and on the air filter. You'll notice a connector hose that runs from the top of the valve cover to the air cleaner cover. This hose transmits crankcase vapors to the air cleaner to be re-ingested by the engine for combustion. If the blow-by in an engine is excessive, the oil vapor transmitted will be heavy enough to form solid deposits within the air cleaner. Our Mercedes-Benz subject vehicle had too much oil within the air cleaner housing, and you can see the recently replaced air filter is already beginning to get saturated.

The amazing thing about all of this is that, except for the noticeably rough idle, there didn't seem to be other obvious symptoms to communicate that this engine was hurting, at least not during our brief test drive. Although our test car had a lot going for it, knowing the real condition of the engine proved very helpful to the buyer of this vehicle, allowing him to negotiate the fairest price for the car. It also gave him a good idea of what future expenditures he might expect, as well as the type of use the car would best be suited for until necessary repairs are performed.

As noted at the outset, any used diesel-powered vehicle should be inspected thoroughly before purchase— their drivetrains and engines most importantly—at least when factoring future repair costs. The cost of an engine overhaul on this vehicle could come close to the selling price of the entire vehicle itself, which is hardly anyone's idea of a screaming deal, even with all the money saved on a more fuel-efficient diesel engine.